Added dotenv and removed flask-pymongo

[santosderek]

Closes #151

• Added removed Flask-Pymongo as a dependency and replace it with PyMongo.
  • We were not using the Flask-PyMongo features and instead were only using PyMongo.
  • Should be cleaner.
• The configuration is now based on environmental variables.
  • The .env file can now be updated to change our config in an easier and simpler way.
• Added try and except blocks to the invitation tests since it would not delete test invitations if an assert failed.
• Replaced all .db. with a single . since flask-pymongo dependency is removed.
• Needed to add a test secret for the unit test to create sessions.
• Updated the .env.example
• Updated readme to reflect new .env file

[santosderek]

Whoever runs this needs to make a .env file from the .env.example that I committed. The readme also shows an example.